Gasification of solid carbon fuels, especially coal, with steam and air or oxygen to produce a synthetic gas (“Syngas”) containing high concentrations of hydrogen and carbon monoxide has been practiced for many years. This classic fuel industry process is known as the water gas reaction, and can be depicted by the idealized equation:C+H2O→CO+H2.The reaction is also called steam gasification reaction, which is highly endothermic, i.e., the reaction absorbs heat from the surroundings. Commercial gasification of solid fuel began during the latter half of the 19th century with the development of the fixed-bed process, wherein a bed of carbonaceous solid, usually coke, is heated red hot by partial combustion, followed by introduction of steam until the endothermic reaction has cooled the bed below the reaction temperature; the bed is then blasted with air to raise its temperature, followed by steam.
During the 1920's, fluidized-bed technology was applied to gasification on a commercial scale to gasify fine fuel particles. A fluidized bed gasifier comprises of a cylindrical vessel in which a certain height of carbonaceous solids particles forms a bed; some gas, mainly steam and oxygen, is provided to the bed through a distributor, also called grid. The lifting force of the gas makes the whole bed materials act like fluids and the gas flowing through the bed forms many bubbles of different sizes. The fluidized bed gasifier is thus also called bubbling bed gasifier. To have the entire bed materials lifted by the gas fed to the gasifier, it is necessary to prepare the fuel particles to the desired sizes. For most bubbling fluidized beds, the particle size used is in the range of 0-10 mm, preferably about 0-6 mm. To obtain the desired size of fuel particles, the raw fuel has to go through the crush and grinding processes. In the fuel preparation process, fine particles, or fines, less than 45 microns in size, can reach as high as 10% of the fuel added to the gasifier. Some of the fines can be entrained by the bubble and have low carbon conversion. The carried fines often cannot be collected by the conventional cyclones. Therefore, it is an urgent issue in the fluidized bed operation to capture the fines, return them to the reaction region, to achieve high carbon conversions for the fines from fuel preparation process.
The fluidized bed gasifier is generally operated at a temperature of about 1,000° C. and various pressures to promote the gasification reaction cited above. To supply heat to the endothermic reaction, some of the carbon in the bed will react with oxygen through combustion reaction. Because the coal particles are fed to the gasifier near the ambient temperatures and suddenly heated to the operating temperature, a lot of fragmentations occur to generate additional fines. These additional fines in the size of 0-50 microns are also hard to collect and recycle. Furthermore, particles collected on the cyclone wall can also be entrained by the gas flow in the cyclone.
Another issue related to the fluidized bed coal gasification is the difficulty with a special type of coal called bituminous coal, which generates plastic materials when suddenly heated up in a fluidized bed to 450° F. to 1,000° F. in the gasifier. This is commonly referred to as caking, which can generate lumps in the bed. The large lumps can cause clinker formation in the fluidized bed when they sink to the bottom of the bed and react with oxygen. The most vicious materials in the caking process are the fines in the coal feed because the fines by nature are generally more quickly heated up than larger particles and therefore have a higher tendency of forming lumps. Reducing caking is desired because it stabilizes fluidized bed operation and resulting in less shutdown due to caking related clinker formation, and broadens the range of coal that can be fed into the gasifier.
Furthermore, the interaction between the bubbles and the particles and that between the gas jetting out of the grid or the distributor can cause particle attrition, which will generate additional fines. The size of the fines is in the range of 0-50 microns. The amount of fines generated depends on the fuel or char particle properties or the initial fines presented in the fuel fed, the design of the gas distributors and the amount of solids particles in the bed.
The fines are the most fundamental issue facing the fluidized bed gasification operation, and effective collection of the fines and recycling them to the gasifier are essential for the fluidized bed gasifier to survive. The fines issue is so severe in the fluidized bed gasifier that one of the fluidization experts (A. M. Squires, 1982, Contributions toward a History of Fluidization, Proceedings of Joint Meeting of Chemical Industry & Engineering Society of China and American Institute of Chemical Engineers, Beijing, September 19-22, pp 322-353) predicted that no fluidized bed gasifier would be built due to the carbon loss with the fine particles. The fines contain between 10-60% of carbon and therefore must be utilized in the gasifier for the technology to be economically competitive. One approach to utilize the fines involves collecting the fines through a collection device such as a cyclone and returning the fines through a dipleg and pressure sealing system to the gasifier. In an ideal situation, the fines collected will be returned to the oxidization region of the fluidized bed, because the reaction rate or the carbon consumption rate in the oxidation region is many times faster than in a reduced or oxygen deficiency atmosphere. Therefore, in the oxidization atmosphere, the carbon particles can be consumed before leaving the bed by the gas lifting forcer. The reaction between the fines and oxygen can provide the heat to the bed for endothermic reactions and syngas generation.
The difficulty in fine particle collection lies in the small size of particle (0-20 microns) and low particle density. Some of these particles will be entrained to the gas and escape the collection. Even if they are collected by the cyclone, it is difficult to return them to the fluidized bed. The most conventional method used in the fluidized bed is an angled dipleg 80 as shown in FIG. 1. In principle, the solids can flow into the fluidized bed from the cyclone 90 by the accumulation of the solids in the dipleg 80 establishing a static head of the solids particles. The salient feature of angled dipleg 80 configuration is that no or very little aeration is required for the solids to return to the fluidized bed 100. However, the fundamental problem with the configuration is that the gas can flow upwards through the dipleg 80, which is detrimental to the cyclone function because it can blow the collected particles into the exit of the cyclone 90 as illustrated in FIG. 2. The inclined dipleg is thus disfavored for this reason (see. e.g. Knowlton, T. M., in Handbook of Fluidization and Fluid-particle Systems, edited by Yang, W.; Marcel Dekker, Inc., 2003). Knowlton teaches a method of using a bypass line and a valve on the line to prevent the large gas bubble rushing up-flow to spoil the cyclone and causes loss efficiency. However, it is impractical both economically and technically to install a valve in the solids return line for the application of gasification because of the high temperature and high pressure operation with solids flows in the line. The fundamental issue is still unresolved in the coal gasification field for the gas reverse flow and carbon losses from the fluidized bed gasifier are still very severe.
One way to avoid carbon losses from the fluidized bed is to adopt an apparatus called loop seal widely used in the circulating fluidized bed boiler, which completely burns the coal to generate steam for power generation or for steam production. An example of such a boiler is given in U.S. Pat. No. 6,237,541 to Allison et. al. To make the solids flowing from the dipleg though the loop seal returning to the bed of the combustor, it is necessary to provide some gas to the loop seal, termed aeration. In the circulating fluidized bed boiler, the gas used for the loop seal aeration can be air or recycled flue gas. U.S. Pat. No. 5,339,774 teaches the techniques to use the recycled flue gas as aeration gas to the loop seal. However, these techniques cannot be easily applied to the fluidized bed gasifier. Because the high carbon content of the gasifier solids and small particle size and low density, any oxygen in the dipleg will cause the carbon to combust in the dipleg to melt the particles and form clinkers in the dipleg. The consequence is the gasifier has to shut down. The aeration gas has to be oxygen free. Also due to the extremely small particles, the added aeration can even cause the cycle to lose efficiency. That is why loop seal has not widely been used in the fluidized bed gasifier. The essential issue here is to ensure that nearly all aeration added has to flow downwards to the gasifier not to the cyclone.
The additional difficulty with the solids collection and recycle system is the pressure fluctuation in the fluidized bed gasifier. These fluctuations can cause the pressure momentously in the bed much higher than the static head of solids in the dipleg, resulting in gas reverse flow from the dipleg to the cyclone. When reverse flow happens, the cyclone loses efficiency. Because such pressure fluctuation can occur frequently, the cyclone efficiency will suffer even with the loop seal. That is one of the main reasons that the fluidized bed gasifier tends to have low cyclone efficiency.
To avoid loss of efficiency in cyclone, the gas reverse flow has to be completely avoided. Furthermore, the cyclone collection efficiency can be improved by forcing a fraction of the gas to flow with the collected solids; in the art of the cyclone collection, it is termed as the gas underflow. Gas underflow will improve the cyclone collection efficiency; and the higher the gas under flow rate, the higher the cyclone collection efficiency. U.S. Pat. No. 5,690,709 to Barnes teaches the art to induce up to 2.5% of the cyclone inlet gas as underflow. However, all those practices are aimed to improving the efficiency of the third stage separator for fluid catalytic cracker (FCC). Where the gas underflow can be relatively easily to induced because the collected solids flow to a vessel or pipe where the pressure roughly equals to or is lower than that at the cyclone inlet. And for nearly all of the applications of underflow cyclones, the gas and solids are introduced to different chambers that are physically isolated using some sorts of walls. For the fluidized bed gasifier, the solids need to return to the fluidized bed where the pressure is about 3-5 pounds per square inch or 20-35 kPa higher than that at the cyclone gas inlet. Because the operating temperature of the gasifier can be as high as 2000° F., it is impractical or economically prohibitive to physically separate the gas solids flow into different chambers as have been done in the third stage separators in FCC. Thus it remains a serious challenge to introduce gas underflow from a cyclone to improve its efficiency in this setting.
In short, although fluidized bed gasifier has been in commercial operation since the 1920's, it remains an unsolved problem for fluidized bed gasifiers that excessive carbon loss occurs from the gasifier as fly ash, and it remains difficult to feed the fines to the gasifier and to handle caking of coal fines.
The present invention provides an apparatus, as well as a method, that improves the fluidized bed operation that solves the above problems.